A number of requirements have been recognized for the use of adhesive pastes to hold foundry shapes in an assembly. These are generally met by adhesive pastes which are based upon polymers having urethane linkages.
These requirements include the need for a separate hydroxyl and isocyanate component each having a viscosity in the range of about 20,000 to about 50,000 centipoise (cps). Similar viscosities for the hydroxyl component and isocyanate component are desirable because the adhesive paste is applied as a mixture by extrusion through a single applicator gun having a mixing chamber a short distance upstream of an outlet nozzle. An adhesive paste in this consistency range can be readily mixed and extruded through such an applicator using pressures in the range of about 20 to 100 psi.
Another requirement is that the adhesive paste have a time period of workability (gel time) of at least two to three minutes. It also is desirable to have a cure time over which the paste reaches at least 70-80% of its final tensile strength in about 10-30 minutes after application to a foundry shape. This intermediate tensile strength preferably is in the range of about 100 to 150 psi, preferably at least about 125 psi.
In addition to holding foundry shapes together, the adhesive paste should also adhere to metal parts and to the foundry mix used in making mold assembly. Furthermore, since such the mold assembly and other parts may be treated by dipping in a water based solution after adhesion and then heated to a temperature of about 275.degree. F. for about one hour after assembly, the adhesive paste must resist breakdown under the conditions of water treatment and subsequent drying.
U.S. Pat. No. 4,692,479 describes such an adhesive paste. Such adhesive pastes have worked effectively, but they are typically catalyzed with lead catalysts and there is an interest in avoiding the use of such catalysts. They also require solvents in the hydroxyl component and polyisocyanate component.